1. Field of the Invention
The present invention relates-in general to an automotive air conditioning system of double function type which has two function cycles, one being a cooling cycle for cooling air by practically using the system as a closed refrigeration system and the other-being a heating cycle for heating air while dehumidifying the same by practically using the system as a heat pump system. More specifically, the present invention is concerned with an automotive dual air conditioning system which incorporates thereinto the principle of the system of double function type.
2. Description of the Prior Art
Hitherto, various types of air conditioning systems have been proposed and put into practical use particularly in the field of motor vehicle. Some are of the above-mentioned double function type.
In long body passenger cars, such as one-box type car or the like, there has been widely used a so-called dual air conditioning system which comprises a first air conditioning unit for conditioning air fed to a relatively front portion of a passenger room and a second air conditioning unit for conditioning air fed to a relatively rear portion of the passenger room.
Nowadays, for obtaining various advantages expected when combined, some of the dual air conditioning systems employ the principal of the system of double function type. That is, for example, the first air conditioning unit comprises a first evaporator constituting part of a closed refrigerant circuit of an air conditioning system of double function type and a heater core fed with engine cooling water, and the second air conditioning unit comprises a second evaporator connected to the closed refrigerant circuit and a condenser connected in series to the second evaporator to serve as a heater.
However, due to inherent construction, the above-mentioned dual air conditioning system of double function type has failed to give users satisfaction particularly in winter. That is, in cold seasons, the system assumes a so-called heating mode. However, the system assuming such heating mode has failed to provide the passenger room with a sufficiently warmed air particularly for a time just after engine starting. In fact, during such time, the temperature of engine cooling water fed to the heater core is still low and the temperature of a refrigerant led to the condenser is low, which causes such undesirable phenomenon. This phenomenon becomes much severe when the engine is of a fuel direct injection type which produces less heat and thus produces engine cooling water having a relatively low temperature.
In order to eliminate such drawback, the applicant has hitherto proposed a measure of providing the system with an extra evaporator for warming, with usage of heat of engine cooling water, the refrigerant just returned to a compressor of the closed refrigerant circuit. With this, the compressed refrigerant led to the condenser can have a higher temperature thereby to promote the heating function of the condenser. That is, the extra evaporator is disposed in a refrigerant flow line extending from the second evaporator to the compressor and warmed by the engine cooling water passing therethrough. That is, the extra evaporator is of a heat exchanger by which a heat exchange is carried out between the returning refrigerant just directed to the compressor and the engine cooling water led from a water jacket of an associated internal combustion engine. This measure is described in Japanese Patent Application 9-90854.
FIG. 12 is a graph showing results of two experiments carried out by the applicant by using above-mentioned measure. The characteristic line indicated by reference "a" shows a temperature of air warmed by a conventional heater core fed with an engine cooling water, while, the characteristic curve indicated by reference "b" shows a temperature of air warmed by a heat pump system which has the extra evaporator incorporated therewith. As is seen from this graph, when the temperature of engine cooling water is below 70.degree. C., the heat pump system surpasses the conventional heater core in warming air. While, when the-temperature of the engine cooling water exceeds 70.degree. C., the conventional heater core surpasses the heat pump system.
The applicant has thereafter proposed other measures for improving the air conditioning system equipped with the extra evaporator, some being to provide a bypass circuit through which the returning refrigerant flows bypassing the extra evaporator, and some being to control the flow of engine cooling water led to the extra evaporator by using ON/OFF valve.